p53 responds to chromosomal DNA damage by activating the transcription of numerous genes that collectively maintain tissue homeostasis. Among the genes most tightly regulated by p53 is PTCH53, a novel target gene that encodes a member of the Patched family of transmembrane proteins. While the mammalian Patched proteins are known to negatively regulate signaling by the canonical Hedgehog pathway, PTCH53 can also strongly enhance the expression of interferon-stimulated genes (ISGs) that contribute to innate immunity. Previous studies have demonstrated roles for p53 in the innate immune responses to viral pathogens, but specific upstream stimuli of these antiviral responses, as well as the downstream intermediates in the ISG pathways, remain incompletely understood. The studies in this proposal are designed to explore mechanistic roles for PTCH53 in the cellular responses to pathogen-associated molecular patterns (PAMPs), in ISG regulation and in the establishment of a p53-dependent antiviral state. The Specific Aims are to determine the mechanisms by which human p53 and PTCH53 enhance ISG expression in cells exposed to structurally diverse PAMPs, and to quantify the contribution of PTCH53 to p53-dependent antiviral defenses, in vivo. This project will comparatively employ genetic models in both human cells and in mice. Successful completion of these Aims will result in the delineation of the fundamental pathways by which p53 responds to pathogens and participates in the cellular defense mechanisms.